The present invention relates to the field of optical signal processing using photonic structures and in particular to optical signal delay elements.
Communications and data processing are increasingly being performed optically. Optical systems are faster than their electrical equivalents and allow for greater data throughput. However, electrical elements are still needed at present to provide data storage and appreciable signal delay. Signal delay is important for a number of data processing applications in data transmission, encryption and processing.
Periodic dielectric structures have been fabricated which exhibit photonic properties analogous in many respects to the electronic properties of semiconductors. A periodic variation in refractive index can give rise to a photonic band structure in which only certain photonic states are allowed.
This is most easily observed in the formation of a photonic band gap. Structures exhibiting a photonic band gap forbid the transmission of light in a particular range of frequencies. Structures of this sort are disclosed in WO94/16345 and WO98/53351.
Photonic bandgap (PBG) structures can be formed by a slab of dielectric material having a periodic array of regions having a different refractive index. Holes can be drilled or etched into the material, or an array of columns can be formed. Alternatively, stacks of dielectric material of alternating refractive index or a series of slots cut into a dielectric substrate can be used to form a 1-dimensional photonic crystal. The properties of the band structure and in particular the response to different frequencies of light are determined by the properties of the materials and by the geometry of the structure.
Examples of the applications of photonic band structures include the formation of waveguides, use in lasing devices, sensors and even in optical multiplexers and demultiplexers.
According to one aspect of the invention an optical system comprises: a modulated optical signal source; an optical input; a delay region having a photonic band structure; and an optical output; wherein the optical input is adapted to couple an input optical signal of a particular wavelength from the modulated optical signal source into a predetermined mode in the delay region such that the group velocity of the optical signal is reduced; and wherein the optical output includes a wavelength selective element to select said particular wavelength.
Modulated input optical signals are coupled into a highly dispersive mode in the delay region in which the group velocity of the optical signal is reduced. The group velocity is the velocity of each optical packet, i.e. the velocity of the optical data. The input signal, which has been delayed and dispersed, is recovered at the output of the device using the wavelength selective element. Input signals with data encoded on a plurality of different wavelengths can be used and each wavelength selected at the output.
Without a wavelength selective element the modulated optical output signal is extremely distorted. The finite packet length of the modulated optical signal gives rise to signal broadening of the transmission wavelength of the optical signal and a band of frequencies will be contained within each packet. The highly dispersive nature of the delay region spreads the frequency content of an input optical packet giving rise to a messy output. The processing performed by the wavelength selective element results in the realisation of a delayed output corresponding to the input signal. The delayed output may be attenuated but it is possible to provide improved transmittance by the use of optical amplification.
Different frequencies within each optical packet will experience dispersion and hence will be delayed and spatially shifted by different amounts in the delay region. The delay region can therefore be considered to process the input signal both spatially and temporally. The recovery of the correct signal can be achieved using either one of these properties i.e. the wavelength selective element may select wavelength spatially or temporally. In the case where the input signal is incident at an angle to normal to the input face of the delay region, the wavelength selective element in the optical output can simply be a correctly positioned output waveguide. This is because the dispersion within each packet will refract different parts of the packet through different angles corresponding to the refraction of different wavelengths. Separate spatial wavelength selection may be achieved through mechanisms such as filtering, refraction, diffraction and interference. Temporal wavelength selection takes advantage of the fact that different wavelengths undergo a different delay. The output signal can therefore be gated to separate different wavelengths.
If a continuous light beam of a single frequency is launched into a photonic band structure and coupled into a dispersion mode only a single wavelength is output, rendering wavelength selection unnecessary. Moreover, a continuous beam has no group velocity as such and so no delay is realised except for that experienced by the phase velocity variation in the photonic structure and material. However, if a modulated optical signal, which necessarily contains a spread of wavelengths, is launched into a photonic band structure the resulting output is so distorted that it is impossible to tell that any part of the signal has been delayed. The output appears to be a meaningless mess. The provision of a wavelength selective element at the output extracts a useful output from the mess.
The temporal and spatial separation of the optical signal also results in power being lost at the output. During post-processing only a fraction of the input signal can be collected and hence loss is experienced. If more loss can be tolerated, it will provide for greater delays.
Preferably, the delay region comprises a first material having a first refractive index including an array of regions having a second refractive index. Preferably, the array extends over a plane in two dimensions. Alternatively, the delay region may be a 1 dimensional photonic crystal formed from a stack of dielectric slabs with alternate slabs forming the array of regions having a second refractive index, or a series of slots cut into a substrate material.
The array of regions having a second refractive index gives rise to a photonic band structure. The characteristics of the band structure are dependent on the geometry and material properties of the array of regions. The frequency response of the delay region is therefore dependent on the geometry and material properties of the array of regions.
Preferably, the array has a low order of symmetry. In particular, the order of rotational symmetry about a point in the array is preferably less than four. A lower order of symmetry gives rise to a less uniform band structure, i.e. a more rapid variation of frequency with wave vector. This gives rise to a greater rate of change of group velocity around the band edges.
Preferably, the array of regions includes one or more defects. This allows the band structure to be tuned more easily as it gives rise to a high Q-factor for the array. The defect could, for example, be a missing region in the array, a displaced region or an enlarged or reduced region within the array. Alternatively, it could be a region within the array having a different refractive index to the rest of the array.
Preferably, the defect is formed from a superposition of two arrays. The superposition of lattices results in a Moire type structure which responds in a similar manner to a set of defects introduced into a single array and is easier to design. Having a set of defects allows light to be coupled into a defect mode more easily than for a single defect. Furthermore, having a large number of defects introduces flat bands in the band structure which allows greater optical delays to be achieved more readily.
Preferably, the finite bandwidth of the optical signal lies in an optimised flat region of the dispersion band, so that most of the delayed signal travels at a similar group velocity. This is also beneficial in reducing the amount of energy lost to other dispersed wavelengths in the optical signal, as they will be removed by a post processing scheme.
Preferably, the first material is silicon nitride or silicon oxynitride.
The delay region may be adapted to allow the transmission of optical signals therethrough, but preferably is adapted to predominantly reflect optical signals of a particular wavelength of operation.
The frequency response of the delay region may be tuned by varying the temperature of the delay region. This causes expansion or contraction of the delay region and hence alters the geometry of the array. Alternatively a piezoelectric material could be used.
Alternatively, the frequency response of the delay region may be tuned by altering the refractive index structure of the delay region. This can be achieved by changing the material composition of the regions, for example when the array of regions is formed from an array of holes in a slab of material, the composition of the material filling the holes can be varied. It can also be achieved by forming either the first material or the array of regions from an electro-optic or magneto-optic material and applying a potential difference or magnetic field across the delay region.
The direction of incidence of optical signals relative to the array can be altered to obtain a different frequency response from the delay region. Preferably, this is achieved by rotation of the delay region relative to the optical input and optical output.
Preferably, the optical system is adapted to cause optical signals from the input to undergo multiple passes of the delay region. The greater the optical path length within the delay region the greater the delay on the optical signal.
The optical system may be adapted such that an input optical signal undergoes a plurality of passes through a delay region. The optical system may also include multiple delay regions. Input optical signals would then pass through each delay region in turn at least once.
Preferably, the optical system includes a delay region and waveguides, the waveguides causing multiple passes of input optical signals through the delay region.
More preferably, the optical system includes two delay regions arranged parallel to one another, each adapted to reflect the input optical signals toward the other, such that, in use, input optical signals undergo a plurality of reflections before reaching an optical output. Preferably, waveguides are positioned between the two delay regions to receive the reflected signals. The delay regions may be stacks of dielectric slabs of alternating refractive index arranged parallel to one another or parallel slots cut into a first material filled with a second material.
The wavelength selective element may be a simple optical filter.
The optical system may be adapted so that the delay region diffracts optical signals as well as reducing their group velocity. The optical output or outputs can then be placed at particular angular positions to receive particular orders of diffraction. The use of a diffracted beam as an output signal provides automatic wavelength selection. The delay region thus acts as the wavelength selective element. This combined functionality is achieved by matching the effective grating pitch of the delay region to the wavelength of operation whilst also coupling the input signals into a suitable mode. The optical input may be arranged at an angle to an input or output facet of the delay region so that the input optical signal is refracted. Owing to the dispersive nature of the delay region, different wavelengths travel at different speeds within it and hence will refract through different angles. Therefore, by positioning the optical output to receive light refracted at a particular angle, wavelength selection is achieved. With the input at an angle to the input facet of the delay region, the input optical signal is refracted at the input facet to spatially separate different wavelengths at the output facet. With the input normal to the input facet but at an angle to the output facet the signal is refracted at the output to angularly separate different wavelengths at the output facet.
The optical system may form part of a phase-arrayed waveguide grating. The delay region is positioned in an input star coupler or a Multi-Mode Interference (MMI) region whilst the outputs of the arrayed waveguides diffract the light into a specific output waveguide in a star coupler type arrangement.
Alternatively, the wavelength selective element may be an optical gate adapted to sample an optical output at different times. The sampling rate is dependent on the bit rate of the input optical signal.
According to a second aspect of the present invention, an optical device comprises: an optical input, a delay region having a photonic band structure and an optical output, wherein the optical input is adapted to couple an input optical signal of a particular wavelength into a particular mode in the delay region such that the group velocity of the optical signal is reduced; and wherein the delay region is adapted to allow the input signal to be coupled into a highly dispersive mode to predominantly reflect the input optical signal from the delay region at the particular wavelength of operation.
According to a third aspect of the present invention, an optical device comprises: an optical input, a delay region having a photonic band structure and an optical output, wherein the optical input is adapted to couple input optical signals into a particular mode in the delay region such that the group velocity of the optical signal is reduced; and wherein the optical device is adapted to cause the optical signals from the input to undergo a plurality of passes through the delay region to thereby increase the optical path length of optical signals in the delay region.
The optical system may include two delay regions arranged parallel to one another, each adapted to reflect the input optical signals toward the other, such that, in use, input optical signals undergo a plurality of reflections before reaching an optical output.
According to a fourth aspect of the present invention a method of applying a delay to a modulated optical signal comprises the steps of:
coupling the optical signal into a particular mode in a photonic band structure; and,
selecting a part of the optical signal output from the photonic band structure, the selection being made on the basis of wavelength.
The step of selecting a part of the optical signal output may include passing the optical signal output through an optical filter, passing the optical signal output through a diffraction grating or passing the optical signal output through an optical gate. Alternatively, the step of selecting a part of the optical signal output may include collecting an angular portion of the optical signal output.